GENETIC TYPE AND THE ENDOCRINES 391 



generation, which is, of course, heterozygous for straight, 

 Stb, in all individuals. Finally, the twisted screw-tail occurs 

 as expected, that is, once in about sixteen individuals. 



On backcrossing the F l with the bulldog, we obtain the 

 following possible combinations of the four tail factors: Ls- 

 Stb = long-straight, Ls-bb = long-bent, ss-Stb = short with 

 simple bend and ss-bb = short screw-tail. As pointed out, 

 long and straight are the dominant, and short and bent the 

 recessive characters, but long dominates short more com- 

 pletely than straight dominates bent. 



A single litter of four backcross bulldog hybrids is illus- 

 trated in plate 79. Figures 1 to 4 show the animals standing 

 on the hind feet, their bodies vertical, and illustrate from 

 life the relative lengths and conditions of the tails. Figures 

 4 to 8 are photographs of the tails, which were placed upon 

 pieces of cardboard to better show their exact outlines. The 

 bulldog-like heads of these four dogs are clearly illustrated 

 in the small photograph (fig. 9). 



Figure 1 in plate 79 has a long-straight tail, Ls-Stb, showing 

 both dominant characters. Figure 2 has a short tail with a 

 slight bend, ss-Stb, thus showing the recessive short condition 

 in combination with the partially dominant straight character. 

 Figure 3 shows a short, simply bent tail of the probable genie 

 composition ss-Stb, the same constitution as the second dog. 

 When animals with these short-bent tails are crossed with 

 those having the complete screw-tail, only half the offspring 

 have screw-tails; the other half show tails that are short 

 with a simple bend. This is a common finding among pure 

 stock bulldogs, many of which are not homozygous for bent 

 and therefore fail to have the perfect screw-tail. Figure 4 

 shows the perfectly expressed screw-tail, ss-bb. Thus this 

 single litter of four backcross bulldog-bassethound hybrids 

 shows all the possible tail conditions for the double factor 

 recessive deformity except the short-straight, ss-StSt, which 

 cannot arise in the backcross, and the long-bent, Ls-bb which 

 is possible but did not appear in this litter. The long-bent 



